Air-conduction type and bone-conduction type HAs (hearing aids) have been known in the past. Insofar as processing methods are concerned, there are analog type HAs (linear-type, non-linear-type (K amplifier), compression-type) and digital HAs. There are a variety of HA types, (box, behind-the-ear, CROS (contra-lateral routing of signal), in-the-ear, bone-anchored, etc.). Kodera describes large desk-top HAs for individual and group training, and smaller personal HAs (see Illustrated Otolaryngology: Kodera K., New Approach 1, Medicalview, 39, 1996).
The digital HA is so constructed that speech as detected by a microphone is first A/D-converted to generate digital data which then is subjected, for example, to Fourier transformation for generating frequency spectral components, after which the amplification factor is calculated, based on the perceptual intensity of the speech, from one frequency component to another. The digital data then is passed through a digital filter for D/A-conversion, to output the resulting analog data to the user's ear. This enables the user to hear a speaker's speech in a noise-reduced state through the digital HA.
A person who has a voice disorder due to a laryngectomy, has lost the phonatory system function through vocal cord vibration, and has dysphonia.
Among the existing speech production substitute devices and methods for people who have had laryngectomies are (1) artificial materials such as the whistle-type artificial larynx using elastic membranes; (2) buzzers, such as the electrolarynx; (3) speech using hypopharyngeal/esophageal mucosa (esophageal/tracheoesophageal speech and tracheophageal speech using voice prostheses); (4) lip-oral electromyography (EMG); (5) phonation/utterance trainers such as CISTA (Computer-Integrated Speech Training Aid); (6) palatographs; and (7) intraoral oscillators.
Digital HAs simply amplify the digital data in each frequency band, while ambient sound is indiscriminately picked up by the microphone, and noise is reproduced as-is, all of which produces a disagreeable feeling in the user. In various types of hearing tests, digital HAs do not show much improvement over analog HAs. Also, conventional digital HAs are not capable of performing processing of detected speech such that the processing differs, depending on the hearing-impaired user's physical impairments (physical state), operating conditions, and/or the purpose for using the HA.
Since speech produced by the above speech production substitutes is not based on vocal cord vibration as it existed before a laryngectomy, the produced voice is inferior in tone quality and does not resemble the user's normal voice.